The present invention relates to an internal combustion engine with combined pressure charging, and, more particularly, to an engine comprising a turbine acted upon by exhaust gas and operable to drive a compressor in an exhaust conduit of the engine, a mechanically-driven supercharger arranged downstream of the compressor in an intake conduit of the engine and connected to the engine via a coupling device, a bypass conduit bypassing the mechanically-driven supercharger, a bypass air flow control device arranged in the bypass conduit, a speed sensor for detecting engine speed, a load sensor for detecting engine load, a control device for controlling the coupling device the bypass air flow control device.
German Offenlegungsschrift No. 3,636,642 discloses a motor-vehicle engine having an exhaust turbocharger and a supercharger driven mechanically by the engine by a coupling device, and a bypass conduit which bypasses the mechanical supercharger provided with a bypass valve arranged therein. At high load and low engine speed, the mechanical supercharger is operated, with the bypass valve being closed. At high engine speed or at low engine speed and low engine load, the mechanical supercharger is stopped by disengagement of the coupling device. In the transitional range between the various operating states, the bypass valve is opened gradually as a function of operating parameters.
This known arrangement has the disadvantage that, due to the high inertia of the mechanical supercharger and its immediately commenced output, very high coupling torques are transmitted upon coupling the mechanical supercharger. These brief power peaks lead to overstressing of the separating clutch or necessitate a very large overall volume. Switching hysteresis thereby results, if, for example, starting from high engine speeds, after overrun operation, charging pressure is again required from the mechanical supercharger.
An object of the present invention is to provide a combined pressure charging internal combustion engine which guarantees as rapid and delay-free a response as possible to load changes of the engine in combination with a high efficiency in all operating ranges.
The foregoing object has been achieved according to the present invention by a pressure sensor for detecting charge-air pressure, wherein a coupling device has a speed-varying device via which a driving speed of the mechanically-driven supercharger transmitted to an engine drive shaft can be varied continuously between a maximum speed and a minimum speed, and the charge-air pressure is adjusted by the control device to a predetermined set point value as a function of the engine speed and the engine load via the bypass air flow control device and via the driving speed transmitted by the speed-varying device to the drive shaft coupled to the mechanically driven supercharger.
Since, in the case of an exhaust turbocharger, there is only gas-dynamic coupling to the internal combustion engine, the charging pressure produced falls sharply in low speed and load ranges. In the event of a jump to full load of the engine, there is a certain time lag in the charging-pressure build-up of the exhaust turbocharger. This leads to delayed engine response characteristics and, particularly in the case of diesel engines, to an increased particulate emission.
In the case of the mechanically driven supercharger, a sufficiently high charging pressure can be built up virtually without a delay, even in the low engine speed range, due to its permanent coupling to the internal combustion engine. The internal combustion engine has to provide the driving power to produce the charging pressure. This results in increasingly high specific fuel consumption figures as the charging pressure increases, compared with an engine pressure-charged via an exhaust turbocharger.
By way of combined pressure charging, a relatively high charging pressure can be achieved even at low engine speeds. Good acceleration characteristics of the engine can be achieved without excessive emission of particulates. The specific fuel consumption in the full-load and upper part-load range is more favorable. The downstream positioning of the mechanically driven supercharger furthermore improves the run-up behavior of the exhaust turbocharger since the air drawn in by the mechanically driven supercharger supplies kinetic energy, as input energy to the existing static pressure reduction in the compressor.
The use of a continuously variable coupling device offers the advantage that the mechanical loading of the continuously engaged mechanically driven supercharger is reduced by continuously high supercharger speeds in circulating-air mode as are its speed-dependent friction power and noise emission. In addition, it is thereby possible to optimize the charge-air pressure over the entire operating range and thus improve the efficiency of the engine.
The present invention offers the advantage that the mechanically driven supercharger can be run up to its operating speed at its lowest power consumption. It also guarantees that the mechanically driven supercharger is operated at its operating speed over as large an operating range as possible, with the result that the necessary charge-air pressure is immediately available, by closure of the bypass valve, in the event of a spontaneous load change.
To increase the engine braking effect, the mechanically driven supercharger is operated at its maximum speed and with the bypass valve closed in the overrunning mode of the engine. In addition, sufficient charging pressure is thus immediately available in the event of an abrupt transition to a load state.
By virtue of the special arrangement of the speed-varying device, the present invention has the lowest torque loading and the highest speed difference between the input and output of the speed-varying device.